In order to increase data rates or frequency efficiency in a mobile communication system, it is essential to use an adaptive transmission scheme for selectively adapting an appropriate modulation and coding method (a transmission method) according to channel environments and received signal qualities. In the adaptive transmission scheme, a mobile station estimates a channel quality depending on the current channel status, and reports the estimate to a base station so that the base station may select a suitable adaptive method. In such adaptive transmission, the channel quality report is very important.
In general, the signal-to-interference-and-noise ratio (SINR) is used as the metric on the channel quality in the report. In detail, the report process includes a step for dividing a range of a predetermined SINR value into regular intervals, and a step for transmitting a binary channel quality indicator (CQI) which corresponds to an estimated SINR value.
It is defined in the IEEE 802.16d orthogonal frequency division multiplexing access (OFDMA) standard that the SINR range between −2 dB and 26 dB is divided into intervals of 2 dB, and a 4-bit CQI sequence represents a SNR value of 16 different values.
For example, when the SINR value measured at the mobile station is given to be 2.5 dB, the mobile station reports, to the base station, a CQI message of “0011” which corresponds to the range in which the 2.5 dB SNIR value belongs. When the base station detects the CQI message, some errors may be generated depending on the channel status. If the base station detects the CQI bit to be “1011” because of an error, it determines the SINR range to be from 18 dB to 20 dB rather than the SINR range of from 2 dB to 4 dB which is indicated by the correct CQI message, and then, it selects a transmission method suitable for the high SINR range, and transmits data to the mobile station. However, the mobile station fails to successfully receive the data from the base station because of low channel quality.
Therefore, a more powerful encoding and modulation method for CQI message transmission is required for reducing the errors on the CQI message. The IEEE 802.16d OFDMA standard transmits a 4-bit CQI message over 48 subcarriers by using a block coding method and an 8-ary orthogonal modulation method.
Referring to FIG. 1, bit error at different position in the CQI bit sequence causes different deviation from the actual SINR. For example, when a base station detect “0011” to be “0010” because of an error in the least significant bit (LSB), the SINR difference between a SINR value of 2 dB to 4 dB to be reported by the mobile station and a SNIR value of 0 dB to 2 dB detected at the base station is about 2 dB and this deviation is small. However, when the base station detect “0011” to be “1011” because of an error in the most significant bit (MSB), the difference between a SINR value of 2 dB to 4 dB to be reported by the mobile station and a SNIR value of 18 dB to 20 dB detected at the base station is 16 dB which is very large.
However, the block coding scheme in the conventional IEEE 802.16d OFDMA standard does not consider different weights on the bits, and hence, an error in the MSB causes a very serious error in SINR report. Also, strong blocking coding and orthogonal modulation increase complexity in realizing a transmitter and a receiver.